Depends on the club. It may not move the dial all that much if you've got the footballing costs of Man Utd, but if your football spend is outside the Sky Six then it could be a big help.
Let's say our revenue is £200m in a season that we finish 7th and qualify for the Europa League. We spent 85% of that, giving us footballing costs of £170m.
The next season in the Europa League, we do pretty well. If you go deep in the Europa League, the prize money alone would be £30m+ - Spurs got £45m for winning it and Man Utd £36m for being runners up last year. And that's before the other additional income streams.
70% of £230m is £161m.
Add in the additional matchday revenue, sponsorships etc and you're in about the right ballpark. It isn't an exact bullet but it can bring in much closer.
However, spend and you have a problem, and teams that qualify for Europe always need to spend.